1. Field of the Invention
The present invention relates to a device for switching communication speed in a modem.
2. Description of the Related Art
In a facsimile protocol such as V.17 and V.29 in the recommendation T.30, a calling side first transmits DIS signal, DCS signal and so on at a low speed of 300 bps, next transmits TCF signal, an image signal PIX at a high speed, then transmits EOP signal again at a low speed.
In the latter half of the image signal PIX RTC signal is added. In the above recommendation, the RTC signal is intended to be detected in an answering side. When the answering side detects the RTC signal, the answering side shifts an apparatus mode from a high speed signal reception mode to a low speed signal reception mode, which enables the answering side to receive the EOP signal that is a low speed signal coming next.
However the RTC signal is hard to recognize because the generation time is short. And it sometimes occurs that the RTC signal can not be detected when a switching device is momentarily interrupted by impulse noise and so on, because the noise resistance of the RTC signal is low. In this case, the low speed is received at the high speed signal reception mode, which remains the problem that the low speed signals after the EOP signal can not be received.
It is an object of the present invention is to provide a communication speed switching device capable of shifting assuredly from a high speed signal reception mode to a low speed signal reception mode.
The object is achieved by a communication speed switching comprising a detecting section for detecting a first frequency used in a low speed reception mode, and a reception mode switching section for switching the mode to the low speed reception mode when the detecting section detects the first frequency for a predetermined interval.